In many applications, typically in various communications systems and especially in multi-carrier modulation systems, e.g. Orthogonal Frequency Division Multiplexing (OFDM), there are requests for non-linear modification of a signal because multi-carrier signals suffer from a high peak to average power ratio. In many cases, such non-linear modifications have to be kept within a certain bandwidth or within certain spectral mask restrictions. One typical example of such non-linear modification is Peak-To-Average Ratio (PAR) reduction. PAR reduction increases efficiency and average output power of a peak power limited Power Amplifier (PA). PAR is more pronounced in multi-carrier systems, e.g. OFDM since a large number of orthogonal, narrowband carriers are used, which when added up coherently give a large PAR. A large PAR ratio brings disadvantages like a reduced efficiency of a Radio Frequency (RF) power amplifier and an increased complexity of analogue to digital and digital to analogue converters.
The objective of peak reduction techniques is therefore to reduce the peak amplitude excursions of the output signal while keeping the spectrum expansion within specified limits, such as spectral mask and adjacent channel power ratio (ACPR) specifications, and keeping in-band error within specified limits, so-called error vector magnitude (EVM) specification.
There are many existing prior art solutions dealing with peak power reduction for multi-carrier signals and signal carrier signals.
In the international patent application WO/2006/068555, non-linear modification of an input signal under bandwidth constraint and spectral mask restrictions is described and can be applied in peak to average reduction systems. In this prior art, non-linear signal conditioning is provided by signal processing in a number of steps, wherein for each step an insertion source signal is provided, which is treated non-linearly to fulfil certain restrictions in bandwidth and spectral mask and for keeping in-band error within specified limits, so-called error vector magnitude (EVM) specification.
Another prior art approach for reducing the peak power of an input waveform is to implement power clipping. In the power clipping approach, whenever the amplitude of the input signal is lower than a predetermined threshold, the input signal is passed to the output unchanged, and whenever the amplitude of the input signal exceeds the threshold, the output signal is clamped to the threshold level. Of course, the clipping operation destroys some of the information contained in the original signal. However, the user should be able to tolerate this loss of information as along as the threshold is kept sufficiently high.
Decresting is another prior art approach for reducing the peak power of an input waveform, while avoiding the overshooting problems caused by the baseband filter in the power clipper. In this approach, which is suggested in the international patent application WO 03/001697, an error signal is created that represents the amount by which the input signal exceeds a threshold. This error signal is then subtracted from the original input signal in order to form a decrested output signal.
Tone reservation is another method used to reduce peak power of a signal, typically used when an input signal is a multi-carrier signal or a multi-tone signal. In this method, described in J. Tellado-Mourello, “Peak to Average Reduction For Multicarrier Modulation” Dept. of Electrical Engineering of Standford University, pp. 66-99, September 1999, the peak power is reduced by selecting or reserving a subset of a plurality of frequencies that make up a multi-carrier symbol. These selected or reserved frequencies are used to create an appropriate impulse function, which is scaled, shifted, rotated and subtracted from the input multi-tone signal at each peak of the input signal that exceeds a predetermined threshold. Thus, one or several peaks may be clipped in this fashion and in one iteration. However, reducing one or more peaks may cause the resulting waveform to exceed the clipping threshold at other positions. Therefore, the process is repeated until a satisfactory peak-to-average reduction is achieved. The impulse function created from the subset of reserved frequencies are usually pre-computed since the subset of reserved frequencies is usually known in advance.
The basic idea of reducing the peak power using a pre-computed impulse function created from reserved frequencies of the multi-carrier signal is attractive and does really reduce the peak power, but for large numbers of carriers or tones, i.e. large number of samples per multi-carrier symbol, and aggressive peak reduction. i.e. peak reduction at low output peak-to-average ratio, the number of peaks gets large. This means that the number of iterations needed to reduce the peak power to a satisfactory level also gets large. This increases the complexity of the implementation and therefore also hardware and power consumption of a transmitter.